5 Answers
5

It means that user id is 1001 but that id has no name assigned. This can happen for example a command running in a chroot with a user defined in that chroot will not necessarily have a name assigned outside the chroot.

It is a uid for users whose name is unknown (e.g. an uid missing in /etc/passwd) or perhaps with strange spelling (like maybe Debian-exim, or a user with a long enough user-name, or for a bad entry in /etc/passwd)

Quote man ps under NOTES section: "If the length of the username is greater than the length of the display column, the numeric user ID is displayed instead."
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Ciro Santilli 六四事件 法轮功Nov 15 '13 at 16:07